Catalytic composites containing molecular sieves are well known in the art and are commonly used in the conversion of a wide variety of hydrocarbons. Recently, L-zeolites in combination with other catalytic components have been discovered to be an effective catalyst for converting light paraffinic hydrocarbons into C.sub.6 -plus aromatics (light paraffin conversion, or LPC process). The C.sub.6 -plus aromatics produced in such a conversion process are valuable as they have a higher octane rating than the feedstock and are useful gasoline blending components. Additionally, the aromatics can be recovered from the product for further processing.
It was observed in U.S. Pat. No. 4,810,683 that clustering of a group VIII noble metal deposited on a non-acidic zeolite used as a catalyst occurs during the carbon burn of a catalyst which has become deactivated through carbonaceous deposits on the catalyst. Since such clustering seriously impaired the performance of the regenerated catalyst (i.e., the carbonaceous deposit-free catalyst), it was imperative to avoid clustering during catalyst regeneration. Avoidance of the high temperature oxidation was not possible since this is the most effective means to remove carbon deposits. However, quite surprisingly it was found that the presence of halogen or halogen-containing compounds in the oxidizing stream prevented such clustering. This permitted the regeneration of catalysts with performance characteristics approaching that of the initial catalysts.
It soon became apparent that other heretofore unrecognized factors also are important in determining the performance characteristics of regenerated catalysts, for the prevention of clustering per se did not necessarily ensure good performance as a regenerated catalyst. Thus, even when regeneration was performed as described in U.S. Pat. No. 4,810,683 the performance of the regenerated catalyst was not necessarily acceptable even though no clustering of the group VIII metal had occurred. Such observations led to the realization that clustering is not the sole factor degrading catalyst performance. Continued work soon afforded the understanding that processes occurring in preparing the finished catalyst played an important role in catalyst performance.
Molecular sieves generally are not used per se in a finished catalyst, especially when the molecular sieve material serves as a support for a metal, but are instead dispersed in a binder. For most industrial applications the discrete particles of molecular sieve materials are too small to be used directly, and so are agglomerated into the larger particles which may be more efficaciously used. These agglomerates are formed by mixing small particles of molecular sieves with a binding agent (binder) where alumina and silica are examples of commonly used binders. The agglomerates consist of a multiplicity of small particles dispersed in a binder and often contain adjuncts such as plasticizers, burnout agents, and extrusion agents, for example. Thus, a finished catalyst of the type under consideration in this application has small zeolitic particles, dispersed in a binder, serving as a support for one or more group VIII metals, generally in a zerovalent state. It is important to note that in a catalyst, at least as first prepared, it is possible to have all of the metals on the molecular sieve surface and none of the metals dispersed in the binder. However, group VIII metals generally are more weakly bound by the molecular sieve than by materials used as binders, and especially by alumina used as a binder. What we have discovered is that even at temperatures as low as 350.degree. C. the metal tends to migrate from the molecular sieve to the binder. Even though the metal may not agglomerate in the binder, the catalytic performance of the composite may be seriously degraded simply because the binder-supported metal may be vastly inferior to the zeolite-supported metal as a catalyst in the process of interest.
What we have found in a catalytic composite of a molecular sieve supporting the metal where the molecular sieve particles are dispersed in a binder is that metal migration from the molecular sieve to the binder at temperatures above 350.degree. C. and up to about 650.degree. C. does not occur, or at least is greatly reduced and inhibited, in the presence of chlorine, hydrogen chloride or a chlorine-containing compound which is a precursor of either of them. Stated somewhat differently, our invention inhibits or even prevents metal migration from molecular sieve particles to the binder in a molecular sieve agglomerate during heating at temperatures of 350.degree. C. and higher, up to about 650.degree. C., in a non-reducing atmosphere by adding chlorine, hydrogen chloride or a chlorine-containing precursor of either of them to the non-reducing atmosphere. Although this invention is clearly related to our earlier one in U.S. Pat. No. 4,810,683 which was directed to regeneration of a catalyst deactivated by deposition of carbonaceous materials by conducting a carbon burn with an oxygen stream containing a halogen or halogen-containing compound in the temperature interval of 300.degree.-600.degree. C., the instant invention is also clearly distinct from that described above in several important aspects. Perhaps most importantly, the invention herein is not limited to catalyst regeneration, and some of its most critical aspects are in fact related to catalyst preparation. Equally important, the invention here is not limited to catalysts having carbonaceous deposits, and in facts some of the applications of paramount importance occur in cases where the catalyst is devoid of carbon deposits.